Gas separation is important in various industries, particularly in the production of fuels, chemicals, petrochemicals and specialty products. A gas separation can be accomplished by a variety of methods that, assisted by heat, solids, or other means, generally exploits the differences in physical and/or chemical properties of the components to be separated. For example, gas separation can be achieved by partial liquefaction or by utilizing a solid adsorbent material that preferentially retains or adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the gas mixture, or by several other gas separation techniques known in the industry. One such commercially practiced gas separation process is thermal swing adsorption (“TSA”). TSA has been an important technique for purifying gases ever since Joseph Priestley separated oxygen from air using solar heat on mercuric oxide. Temperature-swing adsorption is a process wherein a bed of adsorbent is used to selectively adsorb one or more species from a process stream, wherein the adsorbent bed is regenerated in a proceeding step by raising the temperature of the bed, thereby releasing the selectively adsorbed species.
TSA processes, when operated under certain conditions, allow a selective component or components in a gas mixture to be preferentially adsorbed within the pore structure of porous adsorbent materials relative to a second component or components in the gas mixture. The total amount adsorbed of each component in the material (i.e., the adsorption capacity), and the selectivity of the adsorption for a specific component over another component, may often be improved by operating the adsorption step of the process under specific pressure and temperature conditions since both pressure and temperature influence the adsorption loading of the components to a different extent. Species are desorbed because adsorption isotherms are strongly influenced by temperature. Thus, very high purities can be obtained by adsorbing at low temperature (where adsorption is strong) with the release of a strongly held specie being possible by means of high temperatures for desorption. Also, compared to pressure swing adsorption, TSA can be operated in the saturation regime of the isotherm, a significant advantage for capacity and range of utility with zeolitic adsorbents. In TSA processes, heat for the desorption step may be supplied directly by the adsorbent by flowing a hot desorbent gas through the bed, or indirectly through a heating coil, electrical heat source, or heat exchanger which is in intimate contact with the adsorbent.
Various methods of supplying heat to the adsorbent for regeneration have been proposed. These include microwave energy (U.S. Pat. No. 4,312,641), installation of electrical heaters inside the packed adsorbent bed of the adsorber (U.S. Pat. No. 4,269,611) and direct application of electric current to the adsorber for electrodesorption (U.S. Pat. No. 4,094,652). U.S. Pat. No. 5,669,962 discloses a dryer comprised of a shell and tube type adsorber heat exchangers wherein the internal tube surface is coated with fine water adsorbent particles. The dryer can be used in a rapid thermal swing adsorption cycle process. The adsorbent is indirectly heated or cooled by flowing hot or cold feed gas to the separation process through the shell side passage of the heat exchanger. The feed gas acts first as a cold shell side gas in a first absorber heat exchanger then is heated to act as a hot shell side gas in a second absorber heat exchanger undergoing regeneration, and then passes through the tube side of the first absorber heat exchanger where it is dried. Part of the dried gas is used as a purge gas for the tube side of the second absorber heat exchanger. Interchanging the functions of the two adsorber heat exchangers periodically reverses the cycle. The interchange may take place at intervals of from thirty seconds to three minutes. Many of the TSA processes have cycle times significantly longer than this, often as long as 12 hours.
TSA, as practiced, has several disadvantages. For example, in directly heated TSA processes, a hot fluid is typically flowed through the adsorption bed to raise the adsorbent temperature. The greater the temperature rise, the more fluid is needed. The desorbed impurities thus end up dispersed in a large volume of heating fluid, and the large amount of heat that is used to raise the adsorbent temperature is often not recoverable. In some cases, the heat is not recovered because many directly heated TSA systems are operated with long adsorption times (days) and much shorter regeneration times. Finally, the occasional and gradual regeneration gives rise to concentration and flow variations in downstream equipment that can be difficult to manage in an otherwise steady state process plant. In indirectly heated TSA systems, the heat can be supplied with a heat exchanger avoiding dilution of the product with a heated purge gas. However, heat management and the cyclic nature of indirectly heated TSA processes often presents difficulties.
In addition to gas species separations, TSA cycles have been used to thermochemically compress gases. Several heat pump and refrigeration cycles employ a thermochemical compression step (e.g., see Sywulka, U.S. Pat. No. 5,419,156).
While various swing adsorption methods have been commercially practiced over the years there still remains a need in the art for improved swing adsorption methods, particularly when separating CO2 from flue gas and for more efficient use of heat generated in the process.